[A case of carotid superior cerebellar artery anastomosis associated with bilateral hypoplasia of the internal carotid artery represented as the rupture of posterior cerebral artery-posterior communicating artery aneurysm].

A rare case of a variant type of the persistent primitive trigeminal artery associated with bilateral hypoplasia of the internal carotid artery was reported. Left common carotid arteriography revealed a thin left internal carotid artery terminated at the ophthalmic artery. Right common carotid arteriography showed that a narrow internal carotid artery gave off several fine vessels to the cavernous portion, and terminated at the right superior cerebellar artery via the prominent persistent primitive trigeminal artery (PTA). Supraclinoid portions of the bilateral internal carotid arteries were not able to be found. Eventually the entire cerebral hemisphere was supplied by the vertebrobasilar system via the posterior communicating artery. The narrowed supraclinoid portion of the left internal carotid artery was confirmed by surgery of the left posterior cerebellar artery-posterior communicating artery aneurysm. This case may provide an important suggestion about the mechanism of the persistence of the primitive trigeminal artery. In an embryo, at the stage of 5-6mm embryo in size, the posterior communicating artery begins to develop and then PTAs begin to dwindle. If the C1- or C2- portion of the internal carotid artery is gradually occluded, the forebrain, which has to be supplied by the internal carotid system, must be supplied retrogradely by the basilar system via the posterior communicating artery. At this stage, however, the vertebral artery has not developed enough to supply the entire central nervous system. Therefore, the PTA can not regress and this results in the postnatal persistence of the primitive trigeminal artery.